Flame Retardent Zinc Borate

Every year, a substantial amount of zinc borates is produced and used for various applications in industry due to their specific properties e.g. A flame retardant, corrosion inhibitors, smoke suppressant, synergistic effect, anti-bacterial property, good mechanical properties, and high surface area . The utilization of zinc borates in the formulation of polymer-based composites ensures that the composite has a good flame retardant and thermal stability.

There are various zinc borates having different formulas as following: 2ZnO·3B2O3·3.5H2O, 2ZnO·3B2O3·3H2O, 2ZnO·3B2O3·7H2O, 4ZnO·B2O3·H2O, 2ZnO·3B2O3·9H2O, 3ZnO·5B2O3·14H2O, ZnO·5B2O3·4.5H2O, 6ZnO·5B2O3·3H2O, and ZnO·B2O3·2H2O. The molar ratio of B2O3/ZnO and the number of hydrates in zinc borate structures could be arranged by varying the several parameters such as reaction stoichiometry, temperature, time, mixing rate and solid-liquid ratio. Zinc borate with the formula of 4ZnO·B2O3·H2O is preferred in the polymer and rubber industries since it has a high dehydration temperature (approx. 415°C). The use of those inorganic flame retardants has been increasing as they replace the organic chlorine/bromine based additives which have been used in polymers for several decades. 4ZnO·B2O3·H2O is suitable for extrusion requiring high processing temperatures . It is also utilized in paint, electrical/electronic, transportation and building material applications. Recently, luminescence property of zinc borate with 4ZnO·B2O3·H2O structure has been investigated by doping different lanthanides (Eu3+, Eu2+, and Tb3+) . Morphology and particle size of zinc borates, which are used as an additive in polymers, are extremely important. Zinc borate whiskers have improved mechanical properties of polymers since they behave like a fiber and have a high surface area due to low dimensions. In addition, they not only enhance the flame retardant property of polymer but also increase the strength of polymer matrixes .

Production of zinc borate (4ZnO·B2O3·H2O) has been investigated by using several techniques, such as two-step method , surface active agent supported hydrothermal method , wet chemical method and one-step precipitation reaction where stoichiometric ratio of reactants, stirring rate, reaction temperature and time were investigated. While ZnO and zinc salts are used as a zinc source; boric acid and/or borate salts like borax are used in the production of zinc borates. In the wet chemical method, 2ZnO·3B2O3·3.0–3.5H2O was initially produced by the reaction of zinc nitrate (Zn(NO3)2·6.5H2O) and borax pentahydrate (Na2B4O7·5H2O). Then, it was converted into 4ZnO·B2O3·H2O in the second step. Morphology of product had acicular or a rod-shaped structure. The diameter of the rods was in the range of 5–50 nm, whereas lengths of rod-shaped structures were above 1 μm . The synthesis of zinc borate (2ZnO·3B2O3·3.5H2O) was studied using both zinc sulfate and zinc oxide with the help of sonication. The utilization of ultrasonic energy in the reaction has increased the reaction rate inducing a lower reaction time and 90% yield was obtained at low temperatures . In another study, Na2B4O7·10H2O and sodium dodecylbenzene sulfonate were mixed together and Zn(NO3)2·6.5H2O solution was added into that mixture. In this method, higher reaction time was required for a complete conversion. Zinc borate particles had nano-whisker morphology with particle size of 50–100 nm . In surface active agent supported hydrothermal method, ZnSO4·7H2O and surfactant (PEG 300) were mixed, the slurry reacted with Na2B4O7·10H2O for 24 h of reaction time. Nano and microstructured particles are in the form of wire, rod, and lamella-like shapes and microspheres . In the two-step method, production of 4ZnO·B2O3·H2O was carried out by reaction of ZnO and H3BO3 in the presence of seed crystal of 4ZnO·B2O3·H2O, at the boiling point of the mixture. After ZnO mixed with water, the slurry was heated as much as to boiling temperature of the mixture. H3BO3 is added gradually into the reaction mixture .